Synthetic routes, fabrication strategies and engineering solutions leading to new-generation, dynamically-tunable materials are often inspired by biological systems that show a wide range of adaptive responses. Recently, a number of studies have demonstrated that various physico-chemical properties of biological materials that are generally vital for an organism's survival arise from the presence of highly developed surface nanoroughness and exquisite nano-microfeatures. For example, nano-microstructures developed on the surface of gekko feet, lotus leaves, and cicada and butterfly wings enable exceptional adhesive, self-cleaning, water-repelling and photonic properties. Such features have become textbook examples of “smart” biological nanomaterials.
Several efforts have been made to artificially produce nanostructured surfaces to mimic the unique biological structures and their functions. For instance, one effort includes the fabrication of nanostructured surfaces (e.g., regular arrays of well-defined nanostructures with feature sizes of about 300 nm and aspect ratios reaching 100) in silicon using deep reactive ion etching. These structures are stable, and their geometry is highly controlled. Their rigidity, however, makes them unsuitable for use in adaptive materials and devices. For example, they are structurally unchangeable, and thus their geometry is inherently non-responsive.
In an alternative effort, a wide range of artificial responsive materials, mostly involving polymers, have been used. Hydrogels are prominent examples of such materials. In this effort, the nanostructures themselves were defined by the artificially responsive material. Interestingly, the intrinsic flexibility of artificial responsive materials frequently leads to undesired design outcomes, as the features in the soft materials are generally poorly controlled and often susceptible to irreversible collapse.
Accordingly, what is needed in the art are artificially produced nanostructured surfaces that do not experience the problems of conventional structures.